Vacuum fluorescent displays are similar to vacuum tubes. A cathode, or filament/cathode combination, and a grid and plate are mounted in an evacuated glass envelope. The plate is coated with a phosphor. During operation, the heated filament emits electrons which, if unimpeded by the grid, strike the phosphor on the plate, causing visible light photons to be emitted. Vacuum fluorescent dislays have several advantages. They are visible at almost all ambient lighting levels, and from many angles. Power consumption is relatively low and life expectancy is very high. The display can be customized, and virtually any color of display is attainable.
A primary disadvantage of the use of vacuum fluorescent display devices is that three different power supplies may be needed. For example, in the most commonly utilized filament heating system in which the filament is heated with AC power, three different power supplies are needed. The filament requires an AC voltage which is typically in the range of 3 to 6 volts, and the plates and grids require a positive 20 to 40 volts to illuminate and a negative 2 to 6 volts, relative to the filament, for the plate to be cutoff or dark. Optionally, a bias voltage can be applied to the filament in lieu of negative plate and grid bias voltages. The need for these separate power supplies has added to the size and expense of vacuum fluorescent display units and increased the complexity and cost of designing circuits which incorporate such displays. Expensive and bulky transformers are typically required, with associated oscillators and drivers. The driving circuit typically takes up significant space, tends to be inefficient, and may lead to electromagnetic interference (EMI) and in some cases acoustic noise. Such drive circuits also tend to consume relatively large amounts of power, limiting the suitability of vacuum fluorescent displays for portable devices and other applications in which power consumption is critical.